Aluminum XLPE conductors exposed to sea water

[bjenks]

An electrician ran 1000 ft of parallel 350 kcmil Aluminum XLPE cables in a PVC conduit underground. The run was from a non-fused disconnect to several large brine pumps on a platform. About 500 ft in to the run they put a non-wet proof splice kit in. About 7 months ago the customer noticed water boiling in the area of the splice kit and pull box. They then replaced the splice kits with the proper splice kit that was specified by me and is rated for the application. They also replaced the non-fused disconnect with a fused disconnect.

For about 6 months there doesn't seem to be any problems till I got called in today. It seems one of the 6 conductors on both ends of the conductor was dripping sea water out of the aluminum wire ends and of course dripped on the flow meter controls and damaged them. They created a make shift catch basin and it collected several pints of waters and then dried up. They are asking me for recommendations.

My thought is that the bad splice and the heat of the conductor sucked in water between the jacket and the individual strands of wire. The splice got fixed, but water was stuck in the conductor jacket. With the equipment now running, heat has finally cause the water to work its way out. It has since been evaporated. I have proposed an insulation test to make sure the conductor insulation wasn’t damaged when the bad splice was in and a short was forming in the water without any fused protection. If it passes ok and we all agree it was water working its way out of the cable I am worried about one other problem. That is salt water working its way through the whole cable and leaving salt deposit in there for future corrosion. One thought says the aluminum oxide around the strands wouldn’t be harmed by the salts and not to worry about it. Another thought says to replace all the conductors which would be a huge financial burden to the electrical contractor. Please give me your thoughts and anything else we can do to make sure we are OK from here on out.

 

[ScottyUK]

My opinion would be to replace the lot. Better to have the inconvenience when you can at least plan for it, have replacement materials at hand, and personnel available to install them. If you wait you can guarantee it will take itself out of service at the least opportune moment.

The fact that water was boiling would mean it exceeded the temperature rating of the insulation which will probably be 90[°]C for XLPE. Did you identify the source of the heat?


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[bjenks]

The cause of the water boiling was from them having a disconnect at the utility transformer and then running the 1000 ft service lateral out to the marsh area where the pumps are located (underground). When the splices got wet from ground water getting in the hand hole there was nothing there to protect the wires from shorting to the ground water and thus we had a heater in the water. They are going to do a 2500V insulation meggar test this week to see if the insulation is still in working. That is the best they can do without pulling out the conductors and doing a physical inspection.

Since I don't know the alloy in the aluminum I don't know how it stands up to salt water. There are many aluminum allows that handle salt water very well. Ust not sure about this type of aluminum.

 

[TurbineGen]

What voltage are the cables? How close to their current limits are you pushing them?

If this is a 480V installation and larger cables were used to keep the voltage from excessive voltage drop, I might not be so concerned as long as it passes and insulation test and the system isn't too critical. On the other hand, how hot must a cable get to force the water out of it?

Scotty has a good point about when it will fail. They always fail when you need them the most. If this is a critical system that cannot go down for very long, then you are probably best just replacing the cables.

------------------------------------------------------------------------
If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[potteryshard]

There exist a variety of companies which inject a silicone fluid into and through utility underground distribution conductors to purge water and to retard treeing of the insulation. These systems are intended for use on 15 kV insulated cable, but no reason exists to think that it couldn't be adapted to lower voltage conductors and insulation.

It might be possible to purge first with de-ionized water to remove some of the salt and then later switch to the silicone fluid to provide long-term protection. These systems maintain a residual pressure on the fluid and so serve as a positive block against future water infiltration. Given the high ground water level, this might be a good long-term investment.

 

[bjenks]

480V system, there were parallel feeds (basically doubled the size needed) for voltage drop. Nobody know how much current was driven though it during the failure of the splices. However, full load is like 180A which is way below the rating of the parallel feeds.

Potteryshard, I am interested in knowing more about this process. Do you have any links on this or Florida company names?

 

[Zogzog]

These guys can help you, ask for Kevin

http://cbs-florida.com/field_service/index.htm

You will want to do a condition assessment test on your cable to find out what the extent of damage is, a Megger test is only pass/fail, you will need a more comprehensive test to determine if repair and injection or replacement is the best solution.

There are some pros and cons to this process, this thread covers them quite well.

http://www.eng-tips.com/viewthread.cfm?qid=156645

 

[potteryshard]

The biggest name in the business is UtilX; they have quite a bit of information on their web site. If you gooogle fluid injection cable restoration you should be able to find others. For the reasons you mention, this process is common in Florida, and so trained contractors should be available.

Most of this work is done at 15 - 25 kV which of course requires special MV splices to permit fluid flow. At secondary voltages, it should be much easier. It would probably be fairly easy to devise one's own system.

 

[resqcapt19]

The purging process might require that you break open the spice and purge from both directions. The compression of the strands as a result of the splicing process may prevent a successful purge with the splice in place.

 

[bjenks]

Thank you everyone. Very good advice and points.

 

[jghrist]

The responses so far have addressed the insulation, but I would be worried about the aluminum. Underground aluminum connectors that get wet (even in fresh water) corrode and quickly become a white powder. I assume that conductor will do the same thing. I don't know the chemical processes involved, but I have seen the results.

 

[ArcFlashTraining]

The aluminum would corrode faster in the salt water.

http://www.arcflashtraining.net

http://www.arc-flash-training.com

 

[bjenks]

Only through galvanic corrosion, which would require another metal close by for the aluminum to send ions too. Otherwise Aluminum Oxide is a great protector against corrosion. Especially, if a marine type alloy was used. Just learned this recently through exhaustive research.

 

[PHovnanian]

I'd also worry about how the salt might affect the chemistry of treeing in the XLPE insulation.

 

[Zogzog]

Once you have developed water trees silicone injection is not very effective, the cable will need replacement. That is why I suggested a more comprehensive test than just a Megger test.